Cryptosporidium parvum, a common waterborne protozoan parasite, can produce self-limited diarrhea in healthy humans. C. parvum frequently contaminates environmental sources of water, is resistant to chlorination, and cannot be effectively cleared by water treatment plants operating under current standards. C. parvum is responsible for some of the largest waterborne diarrhea outbreaks in the U.S. and has been defined by the Institute of Medicine as one of the emerging microbial threats to the health of the U.S. population. The project is designed to determine the infectivity/virulence of different C. parvum isolates for healthy adults with and without prior exposure, determined by presence of antibody or prior challenge. Screening of 200 healthy adults in Houston has identified an anti-C. parvum seroprevalence of 25%. Healthy adult volunteers were screened thoroughly for immunodeficiency and challenged with various doses of C. parvum. Oocyst excretion, clinical symptoms and antibody response were monitored for 6 weeks post challenge. Anti-C. parvum negative volunteers (IgG, IgM) by ELISA were found to be susceptible to infection with very few oocysts (ID50=132), whereas those who are anti-C. parvum positive (IgG) are much less susceptible to infection with the same strain (ID50=7,500 oocysts). Volunteers with detectable oocysts excreted <103 to 109 oocysts per day and as with other coccidial infections, a trend towards an inverse relationship between the inoculum dose and total oocyst excretion was identified. All infections were self limited and no secondary transmission was documented. Upon re-exposure to oocysts one year later, fewer subjects had detectable fecal oocysts. However, similar rates of enteric illness were identified. An IgM response to primary challenge was noted in the majority of volunteers after primary exposure, whereas IgG was more common upon re- exposure. Interestingly, however, the serologic response was not indicative of either oocyst shedding or the development of illness. In contrast, a significant secretory IgA (sIgA) response was correlated with both parameters and was a useful marker of infection. Inter strain variation in virulence as determined by differences in ID50 has been identified in oocysts isolated from geographically different regions. The virulence, as measured by infectious does (ID50), diarrhea attack rate (AR), ratio of infected to uninfected (IR) volunteers, duration of diarrhea (DD) and relapse rate (RR), produced by three distinct C. parvum isolates was investigated in healthy adults challenged with various oocyst doses. Oocysts used were originally isolated in Ames, Iowa from a calf (Iowa), Maine from a calf (UCP) and Texas from a horse (TAMU). Various doses of oocysts of the trhee strains were given to volunteers to establish an ID50. Depending upon infectivity results obtained in groups of challenged volunteers, the dose range administered was from 10 to 50,000 oocysts. Volunteers recorded the number and form of stools passed and symptoms experienced. All stools passed were collected daily for two weeks, and 24 hour samples were collected three times a week for a total of 45 days. Excretion of oocysts was assessed by direct immunofluorescence. Oocysts used were comparable in age, excystation rates and mouse infectivity. The ID50 for humans differed between isolates: TAMU=9, IOWA=83 and UCP=500 (also in progress), as did the AR (TAMU 86%, IOWA 44%, UCP 54%; TAMU vs. IOWA p=0.045) and the IR (TAMU 13:1, IOWA 3.5:1, UCP 4.5:1). A trend towards a longer mean DD was seen for the UCP (163 hr) compared to the TAMU (94.5 hr) and Iowa (71.95 hr) isolates. The RR was similar for all isolates. Ongoing projects include the study of other C. parvum isolates that are phenotypically or genotypically different, the humoral and cellular immune response to C parvum and the cytokines that regulate them, and the identification of antigens that are associated with protection and the optimization of oocyst detection methods.